1. Field of the Invention
The invention relates in a general manner to a process for depositing a fluorine-doped silica film (SiOxFy) on a surface of a substrate, in particular of an ophthalmic lens.
2. Description of Related Art
Silica-based (SiO2) thin films are widely used in optics and more particularly in the field of ophthalmic optics. Such silica-based thin films are used in particular in anti-glare coatings. These anti-glare coatings are conventionally constituted of the multi-layered stacking of inorganic materials. These multi-layered anti-glare stackings usually comprise one or more layer(s) having a low refractive index in the visible spectral field. Conventionally, these layers of low refractive index are constituted of a silica-based thin film.
The deposition techniques for such silica-based thin films are very diverse, but deposition by evaporation under vacuum is one of the most commonly used techniques. These SiO2-based thin films possess very satisfactory mechanical properties and refractive indices usually of the order of 1.48 for a wavelength around 630 nm.
However, in order to be able to improve the optical performances of the anti-glare stacking on the one hand and to generate novel systems of anti-glare stacking, it would be desirable to be able to lower the refractive index of this low index film while preserving its satisfactory mechanical properties.
In order to resolve this technical problem, it has already been proposed to generate porous silica (SiO2) films, i.e. in which air is imprisoned.
Unfortunately, as well as the complex manufacturing techniques employed, the films thus obtained possess unsatisfactory mechanical properties which are inferior to those of a conventional silica thin film.
Moreover, the use of fluorine-doped silica thin films is known in other technical fields, in particular in the field of microelectronics.
The films are obtained by chemical deposition in the vapour phase assisted by plasma on discs for semi-conductors.
This technique induces a heating of the substrate which is brought to high temperatures, incompatible with the treatment of ophthalmic organic glasses.
Furthermore, these layers pose stability problems. The patent application EP-0.957.017 gives an account of diffusion problems of fluorine to the outside of the fluorine-doped silica film which leads to adhesion problems.
The deposition of a silica film has been proposed in order to prevent this diffusion without, however, giving complete satisfaction.
The article xe2x80x9cCharacteristics of SiOxFy Thin Films Prepared by Ion Beam Assisted Depositionxe2x80x9d by F. J. Lee and C. K. Hwangbo describes thin films of fluorine-doped silicon oxide (SiOxFy). The article describes in particular the deposition of thin films of SiOxFy of a thickness of about 600 nm on glass and silica substrates. The basic vacuum pressure is 1.2xc3x9710xe2x88x924 Pa and the temperature of the substrate is about 150xc2x0 C. The silica is evaporated by means of an electron beam in the presence of oxygen in the chamber and the silicon oxide deposit is bombarded during its formation by a beam of polyfluorocarbonated ions formed by means of an ion gun starting from CF4 gas.
The thin SiOxFy films obtained have refractive indices varying from 1.394 to 1.462 and can be used as optical films.
However, the SiOxFy films obtained by the process of the above article have the disadvantage of taking up water with time and of having an unstable refractive index which increases with time.
The object of the present invention is thus a process for the deposition on a surface of a substrate of a fluorine-doped silica film (SiOxFy) with a low refractive index, stable over time and having mechanical properties at least comparable to the films of the prior art.
According to the invention, the process for the deposition on a surface of a substrate of a fluorine-doped silica film (SiOxFy) comprises:
a) the evaporation of silicon and/or silicon oxide;
b) the deposition of silicon and/or silicon oxide evaporated at the surface of the substrate in order to form on the said substrate surface a silicon oxide film; and
c) the bombardment, during its formation, of the silicon oxide film with a beam of positive ions derived from a polyfluorocarbonated compound or a mixture of polyfluorocarbonated compounds, the process being characterized in that the silicon oxide film is also bombarded, during its formation, by a beam of positive ions derived from a rare gas or a mixture of rare gases.
As indicated above, the deposit of silicon oxide during step b) of the process of the invention is obtained by evaporating silicon and/or a silicon oxide.
A silicon oxide of formula SiOx with x less than 2 or SiO2 may be used. When SiOx with x less than 2 is used, it is necessary that the ambient medium contains oxygen O2.
Of course, a SiOx/SiO2 mixture may be used. SiO2 silica is preferred in the framework of the invention.
The polyfluorocarbonated compound may be a linear, branched or cyclic perfluorocarbonated compound, and is preferably linear or cyclic.
Among the linear perfluorocarbonated compounds, mention may be made of CF4, C2F6, C3F8 and C4F10; among the cyclic perfluorocarbonated compounds, mention may be made of C3F6 and C4F8; the preferred linear perfluorocarbonated compound is CF4 and the preferred cyclic compound C4F8.
A mixture of perfluorocarbonated compounds may also be used.
The polyfluorocarbonated compound may also be a hydrogenofluorocarbon,
preferably selected from CHF3, CH2F2, C2F4H2. The hydrogenofluorocarbon may also be linear, branched or cyclic.
Naturally, a mixture of perfluorocarbonated and hydrogenofluorocarbon compounds may be used.
The rare gas is preferably selected from xenon, krypton and their mixtures. The preferred rare gas is xenon.
During the deposition of the fluorine-doped silica layer, the substrate is usually at temperature lower than 150xc2x0 C., preferably lower than or equal to 120xc2x0 C. and better still varies from 30xc2x0 C. to 100xc2x0 C.
In a preferred embodiment of the invention, the temperature of the substrate varies from 50 to 90xc2x0 C.
The fact that the deposit according to the invention can be performed at a relatively low temperature makes it possible to form thin films on a large variety of substrates and in particular substrates made of organic glass, such as ophthalmic lenses made of organic glass.
Usually, the process of the invention is carried out in a vacuum chamber at a pressure of 10xe2x88x922 to 10xe2x88x923 Pa. If necessary, oxygen gas can be introduced into the vacuum chamber during the deposition of the film.
The fluorine-doped silicon oxide films of the invention usually have a thickness of 10 to 500 nm, preferably from 80 to 200 nm, and the atomic fluorine content of the films is usually from 6 to 10%.
The silicone content is usually of the order of 30% atomic.
The fluorine-doped silicon oxide films obtained by the process of the invention have a refractive index nxe2x89xa61.48, and preferably from 1.42 to 1.45 (for radiation of wavelength xcex=632.8 nm at 25xc2x0 C.).